In the industry of a steel sheet for a can, thinning of the sheet thickness is promoted as countermeasures for cost reduction (weight reduction) of the can and environmental protection. The steel sheet as a material for a can requires a strength corresponding to the sheet thickness. To ensure the can strength despite thinning of the sheet, a yield strength of about 440 MPa or more is required. There is a concern about reduction of the can strength in association with the reduction in sheet thickness. Studies and developments have been made for countermeasures of this concern up to the present. A steel sheet with the steel sheet strength ensured by addition of C of 0.08 mass % or more to increase the strength of the steel sheet, a double reduced steel sheet (DR steel sheet) with the steel sheet strength increased by performing the second cold rolling for work hardening after cold rolling and annealing, and the like have been developed. However, all of them have problems. Since the high C amount of 0.08 mass % or more causes the steel component region of the hypo-peritectic region during solidification in continuous casting, slab cracking occurs due to peritectic reaction. For the DR steel sheet, the strength of the steel sheet is increased. However, this simultaneously causes a decrease in elongation due to work hardening, thus causing the occurrence of cracking during flanging processing. Furthermore, as the lid of a beverage can or a food can, an easy open end (EOE) is widely used. When the EOE (can lid) is manufactured, it is necessary to shape a rivet to mount a tab by bulging processing and drawing processing. The ductility of the material required for such processing corresponds to the total elongation of about 12% in a tensile test.
The material of can body among the three parts of a three-piece beverage can, which is constructed by seaming the lid and the bottom on the can body, is formed in a pipe shape. Subsequently, flanging is performed on both ends of the can body to attach the lid and the bottom by seaming. Therefore, the end parts of the can body also requires a total elongation of about 12%.
For the conventionally used DR steel sheet, the strength can be increased by work hardening. However, at the same time, there has been a problem that the work hardening reduces the total elongation, thus causing inferior processability.
Furthermore, the steel sheet goes through a surface treatment process and is shipped out as a steel sheet for a can. Subsequently, the steel sheet is further subjected to coating, a slitting process, and processing by roll-forming and then welded by a welder. Subsequently, the steel sheet is heated after repair coating of the welded part and goes through necking and flanging, seaming of a bottom lid, internal coating, and a coating-baking process to be a product. Furthermore, the product is filled with its contents and an upper lid is seamed on the product. Subsequently, the product is sterilized by heat in a retort process. When this retort sterilization is performed, it is necessary to keep can strength against an external pressure applied by retort vapors for a can that has a negative pressure inside. When the can strength is lower than the external pressure, dents in the can surface part result. In recent years, to realize can weight reduction taking into consideration the environment, the raw material for a can is thinned. To keep the can strength, a high strength material such as a DR material is used. However, using the thin high strength material reduces shape fixability, thus preventing formation of a cylindrical shape after a roll forming process.
Japanese Patent No. 3663918 discloses a technique of a steel sheet for a can and a method of manufacturing the steel sheet. The steel sheet contains C: 0.01 to 0.10 wt % and Mn: 0.1 to 1.0 wt % and has a Young's modulus E of 170 GPa or less. A roundness of a cylinder portion obtained by forming the steel sheet is less likely to change and the steel sheet is excellent in shape keeping property. Japanese Patent No. 4276388 discloses a technique of a high strength thin steel sheet for a welded can excellent in flange formability and a method of manufacturing the thin steel sheet. The thin steel sheet contains, by mass %, C: more than 0.04% and 0.08% or less, Si: 0.02% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.05% or less, Al: 0.1% or less, and N: 0.005 to 0.02% or less. The sum of solid solute C and solid solute N in the steel sheet is 50 ppm≦solid solute C+solid solute N≦200 ppm, the solid solute C in the steel sheet is 50 ppm or less, and the solid solute N in the steel sheet is 50 ppm or more. The balance is Fe and inevitable impurities.
However, all of the above-described conventional techniques have problems as follows.
In the steel sheet described in JP '918, to reduce the Young's modulus, it is necessary to perform rolling at a transformation point or below in finish rolling of hot rolling. This increases the rolling load and it is difficult to manufacture the steel sheet. Additionally, uniformity of the quality of the material in the width direction decreases considerably. In the steel sheet described in JP '388, to increase the strength, it is necessary to perform primary cold rolling and annealing and then perform secondary cold rolling at a high rolling reduction. Thus, a cost increase is unavoidable. Furthermore, in the DR steel sheet, performing the secondary cold rolling after annealing reduces the total elongation. This does not ensure a total elongation of 12% or more in every part in the width and longitudinal directions of a coil.
It could therefore be helpful to provide a three-piece can and a method of manufacturing the three-piece can which is excellent in workability to form a steel sheet having a yield strength of 440 MPa or more and total elongation of 12% or more, which is preferred as a material for three-piece can body, in a cylindrical shape close to a true circle such that roundness of the can after can forming is 0.34 mm or less.